1,3 propane-diol, oils, and hydrocarbons). Microbial strains have been developed that secrete hydrophobic fuels, similar to constituents of diesel and gasoline, into the culture medium. These fuels can be separated from the aqueous phase without distillation, thereby reducing the energy inputs and facilitating continuous production.

By taking a systems view, metabolic engineering has developed tools for overall biosystems optimization. They are now facilitating the construction of biosynthetic pathways and eliciting novel multigenic cellular properties of critical importance to biofuels production, such as tolerance to fuel toxicity. In the bioprocessing area, the successful development of membrane-based alcohol separations would greatly reduce energy costs from those of the typically used distillation process. Gas stripping, liquid-liquid extractions of secreted fuel molecules, and new adsorbent materials are also being developed that will allow continuous production modes for fermentation-based products. The photosynthetic production of biofuels—the development of low-cost photobioreactors and associated recovery systems for algal biofuel production—is another area of substantial interest that could have major benefits for overall-process economics.

OTHER TRANSPORTATION-FUEL OPTIONS READY FOR DEPLOYMENT BY 2020 AND 2035

So far in this chapter, the committe has focused strictly on certain liquid fuels and considered only biomass and coal as feedstocks, but in this section it explores the advantages and disadvantages of other known transportation-fuel options. The first to be considered is compressed natural gas (CNG). Thereafter, other liquid fuels that can be produced from syngas, including gas-to-liquid (GTL) diesel, dimethyl ether, and methanol, are described. Finally, the technology implications of using hydrogen in fuel-cell-powered vehicles for transportation are discussed.

The earlier sections discussed how coal, biomass, or combined coal-and-biomass gasification produces syngas, which can be converted to diesel and gasoline or to methanol, which can be converted to gasoline. Syngas can also be produced by reforming natural gas. Only if large supplies of inexpensive domestic natural gas were available—for example, from natural-gas hydrates—would the United States be likely to use natural gas as a feedstock for transportation-fuel production. Methanol can be produced from coal synthesis gas and used as a transportation fuel, but the committee judges that the best approach is to convert



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